1. Field of the Invention
The present invention relates generally to light emitting devices and more particularly to light emitting diodes using phosphors.
2. Description of the Related Art
Blue light emitting diodes (LEDs) are used with luminescent materials (phosphors) to produce light emitting devices which emit apparently white light. U.S. Pat. Nos. 5,813,753 and 5,998,925, for example, disclose light emitting devices in which a blue LED is disposed in a reflective cup and surrounded by material including phosphors. A portion of the blue light emitted by the LED is absorbed by the phosphors, which in response emit red and green light. The combination of the unabsorbed blue light emitted by the LED and the red and the green light emitted by the phosphors can appear white to the human eye. Unfortunately, such conventionally phosphor surrounded LEDs frequently emit white light in a central cone surrounded by annular rings of yellow and blue light, rather than emit light in a uniformly white spatial profile.
Lowery U.S. Pat. No. 5,959,316, incorporated herein by reference, discloses that such annular rings of blue and yellow light can result from nonuniformity in the thickness of the phosphor containing material surrounding the LED and consequent spatially nonuniform absorption of blue light and emission of red and green light. In particular, thick regions of phosphor containing material absorb more blue light and emit more red and green light than do thin regions of phosphor containing material. The thick regions thus tend to appear yellow, and the thin regions tend to appear blue. Lowery also discloses that deposition of a transparent spacer over and around the LED prior to deposition of a uniform thickness of phosphor containing material can eliminate the annular rings in the emission profile. However, surface tension makes the shape and thickness of the phosphor containing material, typically deposited as a liquid or paste (solids dispersed in a liquid), difficult to control.
What is needed is a method for uniformly coating an LED with phosphor containing material.
A method for forming a luminescent layer on a light emitting semiconductor device includes positioning a stencil on a substrate such that a light emitting semiconductor device disposed on the substrate is located within an opening in the stencil, depositing a stenciling composition including luminescent material in the opening, removing the stencil from the substrate, and curing the stenciling composition to a solid state. In one embodiment, the shape of the opening in the stencil substantially conforms to the shape of the light emitting semiconductor device, the depth of the opening is about equal to a sum of a height of the light emitting semiconductor device and a thickness of the luminescent layer, and a width of the opening is about equal to a sum of a width of the light emitting semiconductor device and twice a thickness of the luminescent layer.
In one embodiment, the stencil is positioned such that each one of a plurality of light emitting semiconductor devices disposed on the substrate is located within a corresponding one of a plurality of openings in the stencil. Stenciling composition is deposited in each of the openings.
Luminescent layers are formed on light emitting semiconductor devices without contaminating adjacent areas of the substrate and without completely covering, for example, substrate electrical contacts. Thus, wire bonding of such contacts can occur subsequent to the formation of the luminescent layers. In addition, since many light emitting devices can be disposed on a single substrate and simultaneously stenciled with luminescent material containing layers, this method results in a high throughput and is thus cost effective.
In one embodiment, the luminescent stenciling composition includes phosphor particles and silica particles dispersed in an optically clear silicone polymer which can be cured by heat or light. Preferably, the uncured silicone polymer has a viscosity between about 1000 centistokes and 20000 centistokes and the silica particles are fumed silica particles having a surface area per unit mass greater than about 90 m2/g. The silica particles employed in this embodiment impart thixotropic properties to the luminescent stenciling composition. As a result, a layer formed from the uncured luminescent stenciling composition retains its shape after stenciling without collapsing or slumping. Once cured to a solid state, a layer formed from the luminescent stenciling composition exhibits sufficient strength to withstand subsequent processing of the light emitting semiconductor device on which it is formed. In addition, a cured layer of the luminescent stenciling composition formed in accordance with this embodiment is chemically and physically stable at temperatures in excess of 120xc2x0 C. In particular, the cured layer of luminescent stenciling composition will not yellow during prolonged exposure to temperatures between about 120xc2x0 C. and about 200xc2x0 C.
A resulting light emitting device includes a stack of layers including semiconductor layers comprising an active region and a luminescent material containing layer having a substantially uniform thickness disposed around at least a portion of the stack. A surface of the luminescent material containing layer not adjacent to the stack substantially conforms to a shape of the stack. In one embodiment, the light emitting device includes, in addition, an optically transparent layer disposed between the luminescent material containing layer and the stack.
In one embodiment, the active region emits blue light, and the luminescent material containing layer includes phosphors that convert a portion of the blue light to light combinable with a remaining portion of the blue light to produce white light. In this embodiment, the light emitting device emits white light in a uniformly white spatial profile. In particular, the yellow and blue annular rings of conventionally phosphor surrounded white light devices are eliminated.